Early stages of chick somite development

Abstract

We report on the formation and early differentiation of the somites in the avian embryo. The somites are derived from the avian embryo. The somites are derived from the mesoderm which, in the body (excluding the head), is subdivided into four compartments: the axial, paraxial, intermediate and lateral plate mesoderm. Somites develop from the paraxial mesoderm and constitute the segmental pattern of the body. They are formed in pairs by epithelialization, first at the cranial end of the paraxial mesoderm, proceeding caudally, while new mesenchyme cells enter the paraxial mesoderm as a consequence of gastrulation. After their formation, which depends upon cell-cell and cell-matrix interactions, the somites impose segmental pattern upon peripheral nerves and vascular primordia. The newly formed somite consists of an epithelial ball of columnar cells enveloping mesenchymal cells within a central cavity, the somitocoel. Each somite is surrounded by extracellular matrix material connecting the somite with adjacent structures. The competence to form skeletal muscle is a unique property of the somites and becomes realized during compartmentalization, under control of signals emanating from surrounding tissues. Compartmentalization is accompanied by altered patterns of expression of Pax genes within the somite. These are believed to be involved in the specification of somite cell lineages. Somites are also regionally specified, giving rise to particular skeletal structures at different axial levels. This axial specification appears to be reflected in Hox gene expression. MyoD is first expressed in the dorsomedial quadrant of the still epithelial somite whose cells are not yet definitely committed. During early maturation, the ventral wall of the somite undergoes an epithelio-mesenchymal transition forming the sclerotome. The sclerotome later becomes subdivided into rostral and caudal halves which are separated laterally by von Ebner's fissure. The lateral part of the caudal half of the sclerotome mainly forms the ribs, neural arches and pedicles of vertebrae, whereas within the lateral part of the rostral half the spinal nerve develops. The medially migrating sclerotomal cells form the peri-notochordal sheath, and later give rise to the vertebral bodies and intervertebral discs. The somitocoel cells also contribute to the sclerotome. The dorsal half of the somite remains epithelial and is referred to as the dermomyotome because it gives rise to the dermis of the back and the skeletal musculature. the cells located within the lateral half of the dermomyotome are the precursors of the muscles of the hypaxial domain of the body, whereas those in the medial half are precursors of the epaxial (back) muscles.(ABSTRACT TRUNCATED AT 400 WORDS)

Pax3 activation promotes the differentiation of mesenchymal stem cells toward the myogenic lineage

“…During embryogenesis, all skeletal muscles, except for those of the head are derived from transient condensations of paraxial mesoderm, the somites, located next to the neural tube and notochord [22]. Signals from these structures lead to patterning of the somite to form the sclerotome, a precursor for the skeleton, and the dermomyotome, the precursor for both dermis and muscle [22,23].…”

“…Signals from these structures lead to patterning of the somite to form the sclerotome, a precursor for the skeleton, and the dermomyotome, the precursor for both dermis and muscle [22,23]. Cells expressing the transcription factor Pax3 within the dermomyotome migrate medially through the dorsomedial lip of the dermomyotome to form an epithelial sheet, known as the myotome [24].…”

The pattern of MyoD and contractile protein localization in primary epaxial myotome reflects the dynamic progression of nascent myoblast differentiation

“…The somites are the source of all skeletal muscle in the vertebrate body (Christ and Ordahl, 1995) including the satellite cells (Gros et al, 2005), which are responsible for post-natal growth and repair of adult muscle. Limb muscle cells develop from the lateral (hypaxial) half of the somite while epaxial muscle (muscle directly attached to vertebrae) is derived from the medial half of the somite (Ordahl and Le Douarin, 1992).…”

“…The epaxial myotome, which is the first to develop within each somite, involves at least two phases; in the primary phase, the myotome expands along the somite dorso-medial-to-ventro-lateral axis; a subsequent secondary phase involves expansion of the myotome in a superficial-to-deep direction Kahane et al, 2002;Gros et al, 2004). Both phases of myotome growth occur subjacent to the somite dorsal epithelium, the dermomyotome, from which myotomal precursor cells are known to be derived (Christ and Ordahl, 1995).…”

Xenopus bagpipe‐related gene, koza, may play a role in regulation of cell proliferation

“…Despite the complex organization of the mature somite, the unsegmented paraxial mesoderm is a remarkably naive tissue. Maturation of the somite is largely dependent on interactions with adjacent tissues including the ectoderm, notochord, neural tube, and lateral plate mesoderm (reviewed in Christ and Ordahl, 1995). Investigation of the molecular signals controlling somitic development (reviewed by Currie and Ingham, 1998;Saga and Takeda, 2001), suggests that a Sonic hedgehog (Shh) signal emanating from the notochord and floor plate of the neural tube is involved in the induction of sclerotome in the ventromedial somite (Fan and Tessier-Lavigne, 1994;Johnson et al, 1994;Borycki et al, 1998).…”